When I am doing calculations, how much should I round of the masses in the Periodic Table?
My answer is, "Why round at all?"
If you have to look up the mass of H and find that it is 1.008 u, why bother to round? It's only four digits to enter.
If you find a number like 1.007 94 u, then you may want to round before entering it into your calculator.
The quick answer, you may round off so that you still have one more significant figure than required.
If your answer requires 2 significant figures, you could round off 4.002 602 to 4.00. For three significant figures, you could round off to 4.003.
Rounding off numbers that start with 1 is tricky.
For example, a measurement of 99 g has two significant figures. A measurement of 101 g looks like three significant figures, but it has about the same uncertainty (1 part in 100).
You should play it safe and treat a number like 1.008 as if it contained one less significant figure than it actually does.
For example, calculate the mass percent of H in HCl to three significant figures (H = 1.008 u; Cl = 35.45 u).
With no rounding, % H =
By rounding first, % H =
Conclusion: Why bother to round first? Use the numbers you are given and round off only the required answer.